NullReferenceException and events

Question

我对C#还很陌生，我从下面的行中得到了一个NullReferenceException：

CommandEntered(readedLine);

其余的相关代码应该是这样的：

using System;
namespace Eksamensprojekt
{
    class StregSystemCLI : IStregSystemUI
    {
        IStregSystem stregSystem = new StregSystem();
        public event StregSystemEvent CommandEntered;
        public StregSystemCLI(IStregSystem stregSystem)
        {
            this.stregSystem = stregSystem;
        }         
        public void Start()
        {
            Console.WriteLine(stregSystem.activeProducts().ToString());
            Console.WriteLine("Enter your username and then a product ID and hit enter to buy a product. Example: 'username 49340'");
            Console.WriteLine("Alternatively, to buy multiple instances of the same product you can use: 'username amount ID' instead. Example: 'username 10 49340'");
            string readedLine = Console.ReadLine();
            Console.WriteLine("{0}", readedLine);
            if(readedLine != null)
            {
                CommandEntered(readedLine);
            }
        }
    }
}

namespace Eksamensprojekt
{
    class StregSystemCommandParser
    {
        IStregSystem stregSystem;
        IStregSystemUI stregSystemUI;
        public StregSystemCommandParser(IStregSystem stregSystem, IStregSystemUI stregSystemUI)
        {
            stregSystem = new StregSystem( );
            stregSystemUI = new StregSystemCLI(stregSystem);
            stregSystemUI.CommandEntered += ParseCommand;
        }
        void ParseCommand(string command)
        {
            ...lots of code here, that I don't think is relevant to the solution
        }
    }
}

namespace Eksamensprojekt { 
    public class Program 
    {
        static void Main(string[] args)
        {
            IStregSystem stregSystem = new StregSystem();
            IStregSystemUI ui = new StregSystemCLI(stregSystem);
            StregSystemCommandParser sc = new StregSystemCommandParser(stregSystem, ui);
            ui.Start();
        }
    }
}
namespace Eksamensprojekt
{
    public delegate void StregSystemEvent(string Command);
}

我希望以上内容在没有完整代码的情况下可读。我希望发生的是，如果在readedLine中输入了一些内容，并且字符串不是null，我希望它触发一个事件，导致方法ParseCommand（）在类StregSystemCommandParser中运行。如何制作修复NullReferenceException的解决方案？

Answer 1

没有侦听器连接到CommandEntered事件。

调用事件的正确C#习惯用法是：

CommandEntered?.Invoke(readedLine);